1. Field of the Invention
The present invention relates to a center mechanism of a tire vulcanizer for attaching and detaching a bladder for vulcanization-molding to and from a green tire. More specifically, the present invention relates to a center mechanism of a motor driven tire vulcanizer in which a bladder is raised and lowered in the vertical direction so as to attach and detach the bladder.
2. Description of the Related Art
A tire vulcanizer is a machine for heating an unvulcanized tire which is called as a green tire housed in a die from the inside and the outside of the die so as to perform vulcanization-molding, and in general, a extendable and contractible rubber bag formed by a material having elasticity (such as butyl rubber) called as a bladder is attached in the center thereof. This bladder is inserted so as to be along an inner surface of the green tire which is brought in the center of the die of the vulcanizer. A heating and pressurizing medium with a high temperature and high pressure such as steam is supplied to the inside thereof so as to vulcanize the green tire. The green tire vulcanized by heating and pressurizing for a fixed time is to be a vulcanized tire, the bladder is removed and the tire is brought to the next step. Such a bladder type vulcanizer is provided with a device for attaching and detaching the bladder to and from the green tire and the device is called as a center mechanism of the vulcanizer. In this center mechanism, various types of mechanisms are commercialized. The present invention relates to a center mechanism of a tire vulcanizer in which an upper edge part of a bladder is raised and lowered in the vertical direction so as to attach and detach the bladder. As the center mechanism of this type, there are techniques disclosed in the following related documents.
Conventionally, a technique related to a center mechanism for a fluid pressure driven tire vulcanizer is disclosed (for example, refer to U.S. Pat. No. 5,776,507). This center mechanism for the tire vulcanizer is provided with a center post to which an upper clamp ring for retaining an upper edge part of a bladder is attached to an upper end thereof, a first cylinder attached to a lower end of the center post, and a second cylinder arranged in parallel to the first cylinder. The upper clamp ring is raised and lowered by the first cylinder through the center post. The entire bladder is raised and lowered by the second cylinder. This patent document is applied for patent by the present applicant. Although not described in the published document, since there is a need for large force to operate the first cylinder and the second cylinder, a hydraulic cylinder with high pressure is generally used as the first cylinder and the second cylinder.
Here, the first cylinder is attached in series with the center post so as to extend from the lower end of the center post to the lower side. Therefore, the entire length of the vulcanizer is extended and installation height of a die part is suppressed. Consequently, there is frequently a need for forming a pit having predetermined depth in an installation surface of the vulcanizer and installing the vulcanizer at the place. In the center mechanism described in this patent document, the second cylinder is arranged in parallel to the first cylinder. Therefore, the entire length is suppressed in comparison to a center mechanism in which a second cylinder is arranged in series with a first cylinder. However, a countermeasure thereof is not yet sufficient as there is a need for a shallow pit. In order to form the pit, a fundamental work is increased. When the pit is formed, a degree of freedom in changing arrangement of the vulcanizer is decreased.
Since the first cylinder is positioned at a lower part of the center of the vulcanizer, an exchange of the first cylinder is not easily performed in the case of occurrence of oil leakage in the cylinder or the like. There is also a problem that the heating and pressurizing medium with a high temperature and high pressure to be supplied into the bladder and the first cylinder come close to each other so that the life of oil is shortened. Further, since hydraulic pressure is used as a drive source, there is also a problem that a sealing structure thereof is complicated.
As a technique without such problems, the present applicant also proposes a technique related to a center mechanism for a motor driven tire vulcanizer in which fluid pressure driving is replaced by motor driving (for example, refer to Japanese Patent Laid-Open No. Sho 62-6964). This center mechanism of the vulcanizer is provided with a rod (a center post) to which an upper ring body retaining an upper edge part of a bladder is attached to an upper end thereof, a cylinder tube in which a rotational movement body which is formed by a screwing shaft and a screw nut for a ball screw screwed onto the screwing shaft is installed, a motor for rotating the screwing shaft, and a fluid impact absorbing device provided at a lower end of the cylinder tube. By rotating the screwing shaft by the motor, rotational movement of the motor is converted into up and down movement of the screw nut, and the rod is raised and lowered together with the screw nut. At the time of lowering the rod, the screwing shaft is housed in the rod. Therefore, in comparison to the center mechanism for the fluid pressure driven tire vulcanizer described in U.S. Pat. No. 5,776,507, the center mechanism for the tire vulcanizer described in this patent document is capable of suppressing the entire length thereof. Due to the motor driving, there is no problem in the oil leakage, the decrease in the life of the oil, and the complicated sealing structure as in the case of the fluid pressure driving.
However, in the center mechanism for the motor driven tire vulcanizer described in Japanese Patent Laid-Open No. Sho 62-6964, there are the following problems as a driving method is replaced from the fluid pressure driving to the motor driving. In order to explain the problems, at first, a part of operations of the center mechanism for the tire vulcanizer described in this patent document will be briefly described. At the time of performing shaping of a green tire, the upper ring body which is already raised is firstly lowered and stopped at a predetermined position and a brake is put on. This is to avoid a situation that the screwing shaft is rotated by shaping pressure imposed on the bladder so that the upper ring body is raised and a position of the bladder is displaced, and hence the air is mixed into between the bladder contacted onto an inner surface of the green tire with pressure and the inner surface of the green tire. Next, when the shaping is finished, an upper die performs a closing action so as to perform die clamping. Since the shaping pressure is imposed on the bladder even during the closing action of this upper die, there is a need for putting on the brake even during the closing action of the upper die in order to prevent a rise of the upper ring body until the upper die and the upper ring body are brought into contact with each other. The brake is released immediately before the upper die and the upper ring body are brought into contact with each other. Then, the upper ring body is lowered to a press position while being pushed down by the upper die. Here, during the closing action of the upper die, in order to suppress impact force generated at the time of the contact between the upper die and the upper ring body and suppress movement (the rise) of the upper ring body due to the shaping pressure, the brake should be released immediately before (at the very last moment before) the contact between the upper die and the upper ring body. However, setting of the release timing is difficult. Therefore, the center mechanism for the tire vulcanizer described in this patent document is provided with the fluid impact absorbing device serving as a device for absorbing downward impact force in the vertical direction due to the contact between the upper die and the upper ring body.
The problems in the center mechanism for the tire vulcanizer described in Japanese Patent Laid-Open No. Sho 62-6964 firstly include a point that in this center mechanism, a structure is complicated by providing the fluid impact absorbing device. Other problem is that even when the fluid impact absorbing device capable of absorbing the downward impact force in the vertical direction is provided, the brake is a mechanical brake. Therefore, in the case where the brake is not disengaged (or released) at the good timing due to an erroneous operation of this brake or incorrect setting, and even when the screwing shaft is to be rotated through the screw nut by lowering force of the upper die after the contact between the upper die and the upper ring body, the screwing shaft cannot be rotated due to brake force. Consequently, there is sometimes a case where a large overload is continuously imposed on the screwing shaft, the screw nut, the brake and the like so that a screwing part is damaged and the brake is broken.